The present invention relates to an unit constituting evaporative media and to a cooling tower utilizing same.
In a cooling tower, the liquid is distributed on top of a filler material and flows downward, and air is blown across the filler material in counter-flow to the liquid. The resistance to heat and vapor transfer from the wet filler material to the air is dominated by the air boundary layer at the interface of the liquid. It can be shown that skin friction between the air and the liquid is also located at the same boundary layer. Therefore, there is a simple relationship between heat transfer (Q) and skin friction dissipation (SF), which is defined as a Reynolds analogy:
Q/SF=Cpdt/U2
wherein:
Q is the heat flux per unit area of a duct;
dt is the log mean thermal gradient between air and the liquid;
Cp is the heat capacity of the air, and
U is the air velocity.
The above equation can be considered as the theoretical limit on the efficiency of heat transfer. Cpdt can be converted into an enthalpy gradient between the air and the liquid interface. In a conventional cooling tower, when the enthalpy gradient is about 25 kJ/kg and the air velocity is about 2 m/s, the Reynolds analogy will predict Q/SF=20000/4=5000. Actually, in a conventional cooling tower, the ratio of heat transfer to the blower""s work Wb, is considerably smaller, of the order of 200 only. Apparently, only a small fraction of the blown air dissipates as skin friction on wet surfaces.
There are four main reasons for the difference:
a) The air blower""s efficiency is about 50%.
b) In most cases, air exits from the cooling tower at speeds of about 8 m/s, which rejects kinetic energy.
c) The Reynolds analogy relates heat transfer to skin friction at the boundary layer. Thus, the skin friction enhances the heat transfer. In a conventional cooling tower, energy dissipated by wakes and eddies developed in and behind the filler material plays a dominant role. Unlike SF, wake and large eddy dissipation does not enhance the heat transfer from the liquid to the air, and therefore this energy is totally lost.
d) The filler material in cooling towers is usually made of plastic, which does not absorb liquid; it therefore becomes partly dry when the liquid film on the plastic plates does not cover their entire surfaces.
Commonly used filler materials in variable enthalpy (VE) devices are wooden slats and layers of impregnated plastic boards. The distance between the slats is a few centimeters, and the Reynolds number (Re) of the air, related to this distance, is expressed by:
Re=UD/Ni=3000
wherein:
U is the air velocity inside the filler material;
D is the hydraulic diameter of the grooves between the layers of filler material; and
Ni is the kinematic viscosity of the air.
Thus,
D=4A/C,
wherein
A is the area across the flow, and
C is the perimeter of the grooves.
At Re=3000, the wake generated is effective in dissipating the blown air and reduces the efficiency of the cooling tower. In addition, the wet surface area of the plates is usually less than 100 square meters per cubic meter of filler.
In general, an invariant enthalpy (IE) device is characterized by a small thermal gradient between the liquid and the air. Evaporative cooling air enters at, e.g., 30xc2x0 C. and exits at 22xc2x0 C., while the liquid temperature is 20xc2x0 C. The log mean temperature gradient is therefore only 5xc2x0 C., which is equivalent to an enthalpy gradient of 5 kJ between the liquid interface and the air. This gradient is one-fifth of the enthalpy gradient found in a cooling tower. As the liquid temperature is practically constant, IE heat exchangers are usually based on cross-flow arrangements, namely, the liquid flows down while the air is directed to flow horizontally. In a IE heat exchanger, the cross-flow is thermodynamically inferior as compared with counter-flow arrangements, wherein air flows upwards and liquid downwards.
Heat exchangers for IE commonly use cooling pads made of cellulose, which absorb liquid and become wet even when the liquid does not cover the entire area. The cross-flow reduces the water flow rate for evaporative cooling, and thus a substantial amount of liquid is required to thoroughly wet the cooling pad.
In a typical invariant enthalpy (IE) arrangement, a cooling pad is 10 cm wide, 1.5 m long, and 25 m wide, and stands upright as a wall in a greenhouse to be cooled, large blowers force air at a normal speed of 2 m/sec. It can be shown that the cooling capacity of such an arrangement is about 450 kW. The evaporation rate of this system is 0.18 liter/sec. To wet the cooling pads, water distribution on top of the cooling pads should be about 2.5 liters/sec, for a cooling capacity of 450 kW.
In a variable enthalpy (VE) cooling tower, however, a capacity of 450 kW requires a liquid flow rate of 25 liters/sec, which is 10 times more than that required in an IE cooling tower. At this flow rate, the cooling pads will be filled with liquid and the resistance to air flow will be too large. Therefore, in order to obtain the same cooling capacity, the cooling tower should contain about 5 times the area of the cooling pads, since water distribution should be 5 times larger than the area of the water distribution in an IE cooling pad device.
U.S. Pat. No. 3,450,393 (Munters) describes a gas and liquid contact packing for cooling towers in which the spraying device arranged above the packing effects even diffusion of the liquid on the packing. Various modifications of packings are illustrated, so as to assure that the openings in the packings will not become clogged by deposits and scale formation and to prevent flooding of liquid or bridging of liquid droplets within the packings.
It is therefore a broad object of the present invention to provide an evaporative media which will minimize wake dissipation and maximize wet surface area in VE devices.
It is a further object of the present invention to provide improved cooling towers utilizing evaporative media which minimize wake dissipation and maximize wet surface area.
In accordance with the present invention, there is therefore provided a cooling tower, comprising a housing, pressurized liquid distribution means located at the upper portion of said housing; a liquid collection vessel located at the lower portion of said housing; at least one evaporative media located inside said housing between said liquid distribution means and said collection vessel; said evaporative media being constituted by multi-layered, corrugated cardboard sheets arranged to form a cross-fluted structure having wettable surfaces, an array of inlet openings on a first side of said structure, and an array of outlet openings on a second side of said structure substantially opposite said first side; at least one blower located within said housing for producing air flow within the cross-fluted structure of said evaporative media; characterized in that the hydraulic diameter of the flutes of said structure is less than 1.5 cm; the wettable surface area of said structure is more than 250 m2 for every cubic meter thereof, and said blower produces an air flow within said cross-fluted structure of the evaporative media having a Reynolds number of less than 2,000.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.